The disease that is the theme of A Journal of the Plague Year was part of a worldwide pandemic of bubonic plague that cropped up intermittently and in various locales in Europe and Asia over a period of several centuries, beginning in the 14^th century with the so-called Black Death and continuing in Europe until the 19^th century.  The disease was astonishingly virulent; the most conservative estimates credit the Black Death with killing over a third of the population of Europe.

The plague described in this novel was by no means the first infestation in London, the city having experienced a milder outbreak as recently as 1656. As Defoe relates, cases of the plague cropped up in St. Giles in the Fields, a suburban parish of London, in late 1664.  In the absence of adequate sanitation and public health measures or an understanding of transmission vectors for the disease, there was little to impede its spread other than the “social distancing” and quarantining of infected households described in the novel. By its height nine months later, the disease was killing some 7,000 people a week.

The overall death toll from the plague of 1665 is estimated at between 70,000 and 100,000 people, out of a total population of about 460,000, but if Defoe is correct in his claim that upward of 200,000 people had fled the city, the overall mortality rate could have been as high as 40%.  The death rate was surely much higher in the poorer and more densely populated parishes, whose inhabitants did not have the means to flee, as so abundantly depicted by Defoe.

The plague actually raged in London until the spring of 1666, although with greatly diminished virulence, but its eventual disappearance did not mark the end of the misery. The innermost portions of the city were destroyed by a fire that raged in London for five days in September of 1666.